1. Field of the Invention
The present invention relates to a current control apparatus for an inductive electric load used, for instance, in a vehicle-mounted electronic control system, and more particularly, it relates to such a current control apparatus for an electric load that serves to prevent the burnout of the apparatus due to a short circuit of the load or the like for improved current control accuracy.
2. Description of the Related Art
In conventional current control apparatuses for an electric load, there have been known a variety of such ones in which the switching energization rate of a switching element connected between a drive power supply and the electric load is controlled in such a manner that the current detected by a current detection resistor is made equal to a target load current to be supplied. Such a kind of electric load current control apparatus is used to control a current supplied to a linear solenoid that requires a wide range of a variable constant current, or control a current supplied to a fuel injection electromagnetic valve system that serves to hold a fuel injection valve at its open state with a constant low current after rapid opening thereof.
As one example of such electric load current control apparatuses, there has been known one of an internal feedback control type which itself generates a switching drive command in accordance with a deviation between a detected current and a target current command generated by a microprocessor (see, for instance, a first patent document: Japanese patent application laid-open No. H10-225179 (FIG. 2 and Abstract) and a second patent document: Japanese patent application laid-open No. 2000-114039 (FIGS. 1, 6, Abstract, and Paragraph No. 0003)).
In addition, as another example of conventional apparatuses, there has also been known one of an external feedback control type in which a microprocessor merely generates a target load current command, and a deviation integration circuit provided externally of the microprocessor generates a switching drive command in accordance with a deviation between a target load current and a detected current (see, for example, a third patent document: Japanese patent application laid-open No. H5-217737 (FIG. 1 and Abstract)). In the apparatus of the external feedback control type described in the third patent document, the control load of the microprocessor is reduced, but the hardware configuration thereof becomes complicated.
On the other hand, there are a variety of modes about the connecting location of a current detection resistor, but in the conventional apparatuses described in the above-mentioned first and third patent documents for instance, a feeder circuit is arranged between a power supply line connected, through and in the order of a switching element, an electric load and a current detection resistor, to a power supply positive terminal and a ground wire connected to a power supply negative terminal.
In this case, as wiring for the electric load, there are required two pieces of wiring, i.e., positive wiring connected with the switching element, and negative wiring connected with the current detection resistor, and it is necessary to consider the following five modes as abnormal forms or states.
The first mode is a load short-circuit accident in which the electric load itself is internally short-circuited, or a positive-to-negative line short-circuit accident between the positive and negative lines. In this case, it is possible to avoid a burnout accident by interrupting or turning off the switching element when an excessive current or overcurrent is detected by the current detection resistor. In addition, in a transitional time for which the switching element is being turned off, the excessive current to the switching element is limited by the resistance value of the current detection resistor, so a stress on the switching element can be reduced.
The second mode is an in-phase power supply fault accident in which the positive phase wiring and the power supply line are short-circuited with each other. In this case, no energization command is provided to the switching element, or the detection of a maximum current will be carried out in spite of a small energization rate. As a result, a warning indication can be carried out upon detection of an abnormal state, but current interruption cannot be performed. However, since an excessively large load current does not flow, the possibility of causing a burnout accident at once is small.
The third mode is a hetero-phase ground fault accident in which there takes place a short circuit between the positive phase wiring and the ground line. In this case, it is possible to detect abnormality because the detected current abnormally falls or decreases in spite of the fact that an energization command is provided to the switching element, but even if the switching element is interrupted or turned off quickly by the detection of abnormality, an abnormally excessive current flows through the switching element. Thus, this is an abnormal mode of a high probability that there occurs an internal short circuit of the switching element itself, and hence the switching element might subsequently be burned out to be open circuited. This is because the current detection resistor is put into a state that becomes unable to exert its current limiting function.
The fourth mode is an in-phase ground fault accident in which a short circuit takes place between the negative phase wiring and the ground line. In this case, abnormality can be detected due to the fact that the detected current abnormally falls despite the fact that an energization command is provided to the switching element, so the switching element can be interrupted or turned off by the detection of abnormality.
The fifth mode is a hetero-phase power supply fault accident in which a short circuit takes place between the negative phase wiring and the power supply line. In this case, a maximum current is detected in spite of the fact that no energization command is provided to the switching element or the energization rate of the switching element is small. Consequently, a warning indication can be carried out upon detection of an abnormal state, but current interruption cannot be performed, thus resulting in an abnormal mode in which a burnout accident of the current detection resistor or a break or disconnection accident thereof occurs.
In short, the feeder circuit described in the first or third patent document is constructed such that the switching element and the current detection resistor are connected at positions apart from each other with the electric load interposed therebetween, as a result of which there will be a very high probability that when a hetero-phase power supply fault accident or a hetero-phase ground fault accident occurs, protection of these parts is difficult, thus leading to burnout accidents thereof.
On the other hand, in the feeder circuit according to the second patent document, the current detection resistor is connected to an intermediate position between the switching element and the electric load, and hence special wiring for the electric load is the positive phase wiring alone. In this case, there are two abnormal modes which are described below.
The first mode is a load short-circuit accident in which the electric load itself is internally short-circuited, or a hetero-phase ground fault accident between the positive line and the ground line. In this case, it is possible to avoid a burnout accident by interrupting or turning off the switching element when an excessive current or overcurrent is detected by the current detection resistor. In addition, in a transitional time for which the switching element is being interrupted or turned off, the overcurrent to the switching element is limited by the resistance of the current detection resistor, so a stress on the switching element can be reduced.
The second mode is an in-phase power supply fault accident in which the positive phase wiring and the power supply line are short-circuited with each other. In this case, abnormality is detected by abnormal reduction of the detected current in spite of the fact that an energization command is provided to the switching element. As a result, an abnormal state can be detected to make a warning indication but current interruption cannot be performed. However, an excessively large load current does not flow, so the possibility of causing a burnout accident at once is small.
Accordingly, it is ideal from the viewpoint of overcurrent protection to directly connect the switching element and the current detection resistor with each other as in the case of the second patent document, but on the other hand, there exist, as will be described later, two problems; one is great reduction in current detection accuracy, and the other is that a negative voltage is impressed to a differential amplifier for current detection by a commutating diode connected in parallel to the electric load, thus causing a malfunction of the differential amplifier.
In the case of the first patent document, the current detection resistor is connected to the ground line side, so it is possible to achieve current detection with a relatively high degree of precision. However, there exist an individual variation of the resistance value of the current detection resistor and an offset error of the amplifier or an AD converter when the current value is in the zero state. Therefore, the detection values obtained when two large and small currents are supplied to the current detection resistor and the values measured by an external calibration instrument are compared to provide a current proportional constant and an offset error, which are then stored as calibration constants, so that at the time of actual operation, it becomes possible to perform highly accurate current detection by using the calibration constants thus stored.
In addition, as another example of conventional apparatuses, there has been proposed an apparatus of an internal feedback type in which a current detection resistor is connected between a switching element and an electric load, and calibration processing similar to the one described in the first patent document is carried out (see, for instance, a fourth patent document: Japanese patent application laid-open No. 2003-111487 (FIG. 1 and Abstract)). In this case, calibration constants with the use temperature environment being changed are stored, and at the time of actual operation, appropriate calibration constants are utilized in accordance with the actual operation temperature environment.
Accordingly, in the case of the fourth patent document, more sophisticated calibration is performed as compared with the one in the first patent document, but in actuality, it is not a calibration scheme based on the generation factor of the current detection error, so there still remains an error component that changes in accordance with a variation of the drive power supply voltage and the energization duty of the switching element.
Further, as still another example of conventional apparatuses, there has also been proposed an apparatus of an external feedback type in which a current detection resistor is connected between a switching element and an electric load, and a compensation power supply is used to cancel a negative voltage impressed by a commutating diode (see, for instance, a fifth patent document: Japanese patent application laid-open No. H10-39939 (FIG. 1 and Abstract)).
On the other hand, referring to an overcurrent detection circuit upon occurrence of abnormality, in FIG. 1 described in the second patent document, it is constructed such that an output voltage of the current detection circuit is input to an AD converter of the microprocessor, and if an AD conversion value thereof is excessively large, it is assumed that a load short circuit or a ground fault of the positive phase wiring has occurred. In FIG. 3 described in this second patent document, an overcurrent is detected by comparing an output voltage of the current detection circuit with a reference voltage.
Thus, in the case of the method of detecting an overcurrent by the output voltage of the current detection circuit, when the value of a current detection signal changes from 0 [V] to 5 [V] for instance, in order to prevent inadvertent abnormality detection at voltages in the vicinity of 5 [V], a normal signal voltage needs to be in a very low voltage range, and hence there arises a problem that digital conversion accuracy of the AD converter is reduced.
Here, note that the provision of overcurrent detection resistors, the differential amplifier, a comparison determination circuit, and a latch circuit, as shown in FIG. 6 in the second patent document, results in a costly construction which, in particular, requires two current detection resistors that are heat-generating parts, so such a construction is undesirable from the viewpoint of size reduction and power saving.
In the above-mentioned conventional current control apparatuses, for instance, the first or third patent document has a problem that it is difficult to take protective measures against hetero-phase power supply faults or hetero-phase ground faults of the electric load.
Also, in the second, fourth or fifth patent document, protection against short-circuit accidents becomes easy but there is a problem that current detection errors due to the energization duty of the switching element or variation of the power supply voltage have a great influence on current detection accuracy.
In addition, in the first or fourth patent document, calibration means for improving current detection accuracy is disclosed but has a problem that current detection errors caused by the energization duty of the switching element or variation of the power supply voltage can not be corrected.
Moreover, in the second or third patent document, overcurrent detection means is also disclosed but in any case, there is a problem that a detection voltage for normal current is in a low voltage range, so digital conversion accuracy of the AD converter is reduced.